Keying Shallow DOF

I am having trouble trying to pull off a good key in Keylight. In my shot, which has a very shallow DOF, the greenscreen is behind a skyline which is unfocused. I'm having problems trying to get a good key because of this. I tried boosting the screen softness but I still continue to get "eroded" edges and some green spill...does anyone have any suggestions? What's the best way to key unfocused shots in this situation? If possible, is there an easy way where I could add in the shallow DOF after the keying process in post instead?